Color matrix display (CMD) devices, in which a discretely-addressable two-dimensional matrix of picture elements (pixels) is used to create a full-color image are rapidly becoming the dominant technology for both information display and television (TV) applications. Some color matrix displays, such as multi-color organic light emitting diode (OLED) matrix displays and matrix-addressed electroluminescent (EL) panels are self-luminous. Other color matrix displays, such as matrix-addressed liquid crystal displays (LCDs), are not self-luminous. A device, such as an LCD matrix, that is not self-luminous may be called a “light valve”.
Synthesizing full-color images from a limited set of primary colors is a common problem for all CMDs. The trichromatic theory of color vision underlies color synthesis. This theory postulates that all colors are analyzed by the human visual system (HVS) through three different types of response, which correspond to the transformed spectral sensitivities of three different populations of photosensitive receptors in the human retina. Each receptor population is selectively sensitive to optical radiation within a finite spectral bandwidth that approximates separate long-(red), middle-(green) and short-(blue) wavelength response functions. These three response functions are neurally processed and combined in a complex manner to produce what we ultimately experience as color.
According to the trichromatic theory, because the outputs of only three distinct populations of wavelength-sensitive receptors are combined to produce our sensation of the entire spectrum of colors, the appearance of any color can be matched by mixing three appropriately-selected primary color stimuli.
Conceptually, the simplest form of additivity involves directly superimposing three differently-colored beams of light or colored images. Some full-color electronic displays combine light from primary-colored luminous sources. For example, some projection displays designed for group viewing superpose three projected primary-color images. The optical systems for such projection displays tend to be large and cumbersome. Precise alignment of the three projected images is often difficult to achieve. Moreover, most displays of this type tend to be low in luminance.
Direct-view displays that attempt to superimpose three primary-color images have the added problem that manufacturing yields and device reliability can be undesirably low in devices that require fabrication and alignment of three independently-addressable primary color pixel arrays. This raises manufacturing costs. Further, direct-view displays which utilize spatial superposition are prone to viewing parallax at off-axis observation angles. Viewing parallax results in the perceptible misregistration of the three primary color pixel arrays and becomes increasingly apparent as the pixel dimensions relative to the thickness of the individual color pixel arrays decreases (e.g. high-resolution color displays) and/or the off-axis observation angle is increased.
Some displays exploit the fact that the HVS is quite limited in both spatial and temporal resolution. Integration occurs beyond the spatial and temporal resolution limits of the HVS. Therefore, spatial or temporal patterns composed of three appropriately-selected primary colors are sufficient for producing a full range of colors when the spatial or temporal frequencies of the patterns exceed the respective resolution limits. This permits spatial-additive and temporal-additive color synthesis (see Wandell & Silverstein, Digital color reproduction in S. Shevell (ed.) The science of color, 2nd edition, Optical Society of America, Washington D.C. (2003) pp. 281-316 for coverage of both the basics of color vision and color synthesis in imaging systems).
The HVS can be modeled as having a channel structure in which luminance and chromatic channels are separable. FIG. 1 provides a schematic representation of the opponent-process visual channels from the retinal photoreceptors through the early neural processing stages. The low-level visual channels are organized into two opponent chromatic channels, a R-G channel and a yellow Y-B channel, and a luminance channel formed by the sum of R and G photoreceptor outputs. In this representation the short-wavelength or B photoreceptors do not contribute to the luminance channel. FIG. 2 shows the photopic luminous efficiency function of the HVS and further illustrates the very small contribution of short wavelengths to image luminance.
FIG. 3 shows results from psychophysical investigations of the resolution of the luminance (intensity) and chromatic visual channels (R-G and Y-B) of the HVS. It is clear from these data that the luminance or intensity channel is responsible for most of the spatial resolving capability of the HVS. The chromatic channels, and particularly the short-wavelength or B input, manifest dramatically reduced spatial resolution relative to the luminance or intensity channels.
The temporal resolution limits of the HVS manifest a similar separability of luminance and chromatic visual channels. FIG. 4 shows the results of Varner Temporal sensitivities related to color theory, Journal of the Optical Society of America A (1984) pp. 474-481 which examined the temporal contrast sensitivity functions for temporal modulations in luminance and chromaticity. Temporal resolution is much higher for time varying modulations of luminance than for chromaticity. Thus, it is clear that temporal sequences of different chromaticity or color imaged on the same region of the retina integrate to produce a mixture color. Such temporal integration takes place even at low to moderate temporal frequencies.
Patterns of spatially separate image points of different color, if small enough and viewed from a sufficient distance, cannot be individually resolved by the HVS and integrate spatially to produce a color which is equivalent to a mixture of the image points within a small projected region of the retina. The color produced by spatial synthesis is effectively the same as that produced by direct superposition.
X-Y spatial-additive color synthesis has by far been the most successful method for producing multi-color displays and is exemplified by the shadow-mask color CRT and the color liquid crystal display (LCD). A common feature of such devices is the X-Y spatial mosaic of color primary elements. Most color pixel mosaics consist of a repetitive geometric pattern of R, G and B sub-pixel elements, although some mosaics with an additional fourth color, such as white (W) or yellow (Y), exist for specific applications.
Self-luminous devices of this type require separate emitters which each produce light in one of the wavelength bands of the primary colors. These separate emitters are arranged to produce the color matrix pixel pattern. In non-self-luminous devices, such as matrix-addressed LCDs, a color filter array is imposed over the matrix of individual light valves and aligned such as to produce a mosaic of color pixels.
Many color pixel mosaics can be used. These include:                the RGB vertical stripe mosaic (see FIG. 5A),        the RGB diagonal stripe mosaic (see FIG. 5B),        the RGBG quad mosaic (also called the Bayer filter pattern) (see FIG. 5C),        the RGB delta-triad mosaic (see FIG. 5D),        the RGBW quad mosaic, and        the RGBY quad mosaic.        
FIGS. 6A through 6C, and FIGS. 6D through 6F show the mosaic pattern, associated spatial Nyquist limits and associated fixed-pattern noise modulation spectra of the RGB vertical stripe and RGB delta-triad mosaics, respectively. The spatial Nyquist limits define a two-dimensional spatial frequency baseband which indicate the maximum spatial frequencies which can be rendered with a color pixel mosaic without aliasing artifacts. This is an effective descriptor of the resolution of the mosaic. The fixed-pattern noise modulation spectra indicate the residual luminance modulation as a function of spatial frequency which is produced by the mosaic when rendering a white flat-field image (i.e., all of the color sub-pixels are activated). Thus, this is an effective descriptor of visual noise contributed by the color pixel mosaic itself. A comparison of FIGS. 6B and 6E shows that the resolution of the RGB delta-triad mosaic is substantially higher than that for the RGB stripe mosaic as evidenced by the larger baseband; and the fixed-pattern noise contributed by the RGB delta-triad mosaic is much lower than that produced by the RGB stripe mosaic as evidenced by the higher spatial frequencies and lower modulation values for the RGB delta-triad mosaic.
The color pixel mosaic, the addressing algorithm which maps image content to the matrix of primary color sub-pixels, and HVS characteristics interact in a complex manner to determine overall CMD image quality. Although X-Y spatial-additive color synthesis has been an extremely successful approach for generating full-color images X-Y spatial additive color synthesis sacrifices display resolution. The use of an RGB pixel mosaic consumes three sub-pixels for each full-color image pixel. This reduces the spatial imaging potential of the display. While optimized pixel addressing algorithms and image processing methods can somewhat reduce the impact of this loss of spatial resolution on image quality, the trade-off between spatial resolution and color is a significant limiting factor.
Another limiting factor of X-Y spatial-additive color synthesis is the fixed-pattern noise produced by the mosaic of primary color sub-pixels. This modulation arises due to the differing luminous proportions of the primary color sub-pixels required to produce specified mixture colors such as a balanced white. In general the B sub-pixel is the largest contributor to the fixed-pattern noise of color pixel mosaics as short-wavelengths contribute little to luminance, and the luminance required of B sub-pixel elements is low compared to the surrounding R and G elements for most mixture colors. The level of fixed-pattern noise is dependent upon the particular geometry of the various color pixel mosaics and varies as a function of spatial frequency.
Temporal color synthesis, or field-sequential color can be used to generate full-color imagery without the loss of spatial resolution. Field-sequential color has been employed in projection display systems. There is also a long history of attempts to use the field-sequential color approach for synthesizing full-color images in TV and information displays. Despite these persistent attempts at commercialization, field-sequential color display technology met with only marginal success until advances in display electrical and optical components made it practical to make cost effective systems with sufficient temporal bandwidth and optical efficiency to be commercially successful.
Two limitations of temporal color synthesis are that: residual luminance differences between the time-varying color components can produce observable luminance flicker for temporal frequencies at or beyond those at which effective chromatic integration has taken place; and temporal synthesis assumes that the time-varying color components are imaged on the same region of the retina, an assumption which is often violated in the presence of head and eye movements. Eliminating flicker in a field-sequential color display utilizing R, G and B color fields generally requires a high field rate. This substantially increases the temporal bandwidth requirements of the display system.
Relative movement between the displayed image and the retina of the display observer is harder to correct for. This relative movement may arise either as a result of dynamic motion of the image or head and eye movements of the observer. In either case the result is that the time-varying color components are no longer imaged on the same region of the retina and the observer experiences “color break-up.”
Silverstein et al., U.S. Pat. No. 5,642,125 and Sprague et al., U.S. Pat. No. 5,315,418, disclose color displays which combine a red-green image component with a blue image component to yield color images.
There is a need for improved color display technology.